A person in a manual wheelchair tends to lean forward when going up a sloped surface and lean backward when going down a sloped surface. This is a natural movement of the body done in order to adjust the position of the center of gravity of the user and the wheelchair so that the user keeps his or her balance in the seat. A person in a powered wheelchair, however, cannot lean to compensate for slopes in a surface because the seat of the power wheelchair follows the slope of the surface. Thus, when a power wheelchair goes down a slope, the user is effectively falling forward out of the seat. If the user is able, he or she pushes back on the arm rests of the wheelchair to stabilize himself or herself. However, even pushing on the arm rests to maintain balance can be difficult because the user may need to use at least one hand to control the joystick that operates the powered wheelchair. Thus, in some cases, the user of a power wheelchair may need to wear a chest harness to keep the user from falling out of the seat when traversing downward sloped surfaces. Moreover, when going uphill in a powered wheelchair, the user may be adequately stabilized because the seat is tilted backwards, but the user's frame of reference and equilibrium may be compromised because the user is forced into a reclined posture and line of sight.
There are conventional power wheelchairs that have seats that can be tilted backward and forward to compensate for uphill and downhill slopes. However, the user often will not be able to gauge how far to tilt the seat to compensate for the slope and, therefore, will not be able to find the best orientation for the seat, especially if the user is trying to drive the wheelchair at the same time the user is adjusting the angle of the seat. Moreover, even if the seat of the wheelchair can be tilted to maintain the user in an upright position, the tilting of the seat typically moves the center of gravity (“CG”) of the user and seat to a less stable location. For example, with respect to mid-wheel drive power wheelchairs, the preferred location of the CG for purposes of stability is directly over the center of the drive wheels. However, when the seat of a mid-wheel drive wheelchair is tilted backward, the CG moves to a position behind the rear of the drive wheels. The seat is less stable when the CG is located behind the rear of the drive wheels.
Furthermore, the ability to tilt the seat backward or forward does not help stabilize the seat or improve the orientation of the seat when the wheelchair is traversing a side slope or uneven obstacle.